Electronic random sequence generators have been in use since about the mid-1950's. The major applications have been in tactical communications as well as in guidance, anti-jamming and anti-interference systems. The two major categories of random sequence generators are analog circuits, including noise diodes, and digital circuits, including linear sequence generators.
In a typical analog random sequence generator, a thermal noise source, such as a temperature limited diode, produces a train of pulses in which the intervals between pulses follow a random distribution. The randomness of the occurrences of such a train of pulses may be employed in any one of a number of ways well known in the art to provide a random sequence of data. For the purposes stated, sequence generators using noise diodes provide the beneficial aspects of true randomness: non-repeatability and non-predictability. However, their disadvantages include their large size, the difficulty in screening noise diodes, their susceptibility to an RF environment and their difficulty in testing.
Digital random sequence generators typically employ linear sequence generators which consist essentially of a ring-connected shift register, in which the input signal to the device at any time is determined by the logical combination of the output signal and the signal present in one or more intermediate positions of the device. Appropriate choices of the stages and the logical operations will cause the device to generate a pulse sequence which repeats only after a known interval and has a random distribution over this interval. Because its pulse sequence is periodic, and therefore predictable, such a generator is referred to for convenience as a pseudorandom sequence generator.
It is the purpose of the present invention to provide an apparatus for generating an unpredictable stream of digital data. It is an object to extract the useful properties from linear sequence generators, while eliminating their undesirable properties. The useful properties of linear sequence generators include the mean, the variance and the distribution which satisfy statistical means of measurement for a test of randomness. The undesirable properties, periodicity and predictability, can be combined in one parameter, since periodicity is a necessary condition for predictability. Therefore, the undesirable properties of a linear sequence generator may be eliminated by breaking the periodicity.